The present invention relates to a chemical process for coating magnesium and its alloys, and to a coating so formed.
With the increasing awareness of fuel consumption and human ecology, a global commitment has been made to reduce vehicle mass through application of lightweight materials. Magnesium is the lightest structural metal with the highest specific strength and is the eighth most abundant element on the earth. Many researchers and developers have looked to magnesium to provide a solution for vehicular mass reduction for the automotive, aircraft and aerospace industries. However, challenges exist owing to its low corrosion and wear resistance. To achieve the necessary mass reductions, various coating technologies have been applied to enhance the corrosion and wear resistance of magnesium alloys. To date, no coating technology provides a solution that satisfies the combination of functionality, cost, scalability and environmental concerns. Development of a high volume, environmentally friendly, low cost and mass production scaleable coating process to increase the corrosion and wear resistance of magnesium remains a challenge. Conventional coating technologies are briefly summarized below.
Conversion coatings, the most commonly used type of coatings, contain hexavalent chromium, a highly toxic carcinogen. Conversion coatings alone do not provide sufficient corrosion and wear protection for magnesium alloys in harsh service conditions. Conversion coatings are generally used as an undercoat.
Anodizing is a process that does not provide sufficient corrosion resistance without further sealing because the coatings produced are comprised of a thick porous layer over a thin continuous barrier layer. The coatings produced are brittle insulating ceramic materials, which limits their use in applications where electrical conductivity or load-bearing properties are necessary. High energy consumption is another drawback to this process.
Gas-phase deposition processes require large capital investment and cannot uniformly coat complex shapes due to their line of sight nature. The corrosion, adhesion and wear properties of these coatings on magnesium alloys have not been well documented.
Organic coatings alone do not have sufficient corrosion and wear resistance to protect magnesium for use in harsh service conditions. They are typically used as top-coats and must be applied in multiple layers due to difficulties in achieving uniform pore-free coatings.
Electrochemical coating processes are available for plating of magnesium alloys. These processes are alloy specific and do not work well on alloys with high aluminum content. Direct electroless nickel plating and zinc immersion are two types of electrochemical coating processes.
Direct electroless nickelplating is limited by the short lifetime of the plating baths, the toxicity of chemicals used in the pretreatment process and the narrow operating window required for optimum coatings.
Direct electroless nickel plating comprises a pretreatment process in which electroless nickel is plated directly onto magnesium alloy AZ91 die castings, developed by Sakata et al.(1). In general the pretreatment is as follows:
Pretreatxe2x86x92Degreasexe2x86x92Alkaline Etchxe2x86x92Acid Activationxe2x86x92Alkaline Activationxe2x86x92Alkaline Electroless Nickel Strikexe2x86x92Acid Electroless Nickel Plating.
This process has been criticized (2) for using an acid electroless nickel treatment that can result in corrosion of the underlying magnesium if any pores are present in the nickel strike layer. A simpler process has been developed by PMD (U. K.) Limited (see references 3, 4, 5). The basic sequence of this pretreatment is as follows:
Pretreatxe2x86x92Alkaline Cleanxe2x86x92Acid Picklexe2x86x92Fluoride Activationxe2x86x92Electroless Nickel Plating.
The authors determined that the etching, conditioning and plating conditions had a large effect on the adhesion obtained. An insufficient etch or fluoride conditioning resulted in poor adhesion. It was also determined that using hydrofluoric acid for conditioning led to a wide plating window while ammonium bifluoride resulted in a much narrower (pH 5.8-6.0 and temperature=75-77xc2x0 C.) window for acceptable adhesion. The chromic acid treatment was found to heavily etch the surface and leave behind a layer of reduced chromium. The fluoride conditioning was found to remove chromium and control the deposition rate by passivating the surface. The passivating effect of fluoride was also exploited in the plating of magnesium alloy MA-8 (6). In this case the nickel plating bath contained fluoride to inhibit corrosion of the substrate during plating. The authors report strong adhesion of the nickel film however, the bath life is too short to be industrially applicable. The addition of a complexing agent, glycine, was shown to improve the stability of the plating bath. Another proposed process (7) involves treatment of the sample with a chemical etching solution containing pyrophosphate, nitrate and sulfate, avoiding the use of toxic chromium ions. The process sequence is as follows:
Chemical Etchingxe2x86x92Fluoride Treatmentxe2x86x92Neutralizationxe2x86x92Electroless Nickel Plating.
The electroless nickel plating bath does not contain any chloride or sulfate. The plated samples achieved have high adhesion and corrosion resistance. One obstacle to coating magnesium with nickel is that most conventional nickel plating baths are acidic and can attack or corrode the magnesium surface. This problem has been addressed by the development of an aqueous acidulated nickel bifluoride electroplating bath that contains a polybasic acid (8). This bath has been shown to not corrode magnesium.
Zinc Immersion Processes (see references 9a and 9b) are limited by the poor uniformity of the zinc undercoating produced as well as the need for a copper cyanide strike prior to any further plating. The chemicals required for zinc immersion processes are extremely toxic. The zinc immersion pretreatment process has been criticized for the precise control that is required to ensure adequate adhesion. In many cases non-uniform coverage of the surface is seen with spongy non-adherent zinc deposits on the intermetallic phase of the base alloys (1). The copper cyanide strike that must follow has also been criticized for a number of reasons (1). The first is that it is an electroplating process, which means that it is more difficult to coat complex shapes. Copper deposits slowly in the low current density areas, which allows attack of the zinc by the plating solution. This in turn allows attack on magnesium by the plating solution resulting in non-adherent copper depositing by displacement directly on the magnesium surface. The deposits in these areas are porous and have poor corrosion resistance. The second criticism levelled at the copper cyanide plating process is the high cost treatment of waste generated by the use of a cyanide containing bath. A patented methodology (10) attempts to improve this process by eliminating the copper cyanide step from the pretreatment process. The copper cyanide electroplating is replaced by a zinc electroplating step followed by copper deposition from a pyrophosphate bath after the zinc immersion. This patent claims that by creating a uniform zinc film of at least 0.6 micrometers in thickness, adherent plating films can be obtained on any magnesium alloy using the disclosed process. The zinc electroplating step can occur simultaneously with the zinc immersion process or in a separate step. The process is as follows:
Degreasexe2x86x92Alkaline Cleanxe2x86x92Acid Cleanxe2x86x92Activationxe2x86x92Zinc Immersionxe2x86x92Zinc Electroplatexe2x86x92Copper Plating.
A number of processes based on the zinc immersion pretreatment process have been developed. The three main processes are the Dow Process, the Norsk-Hydro process and the WCM Canning Process (1, 11). One criticism of all of these processes is that they do not produce good deposits on magnesium alloys with an aluminum content greater than 6-7% (12). The general pretreatment sequence for each of these is outlined below for comparison (1, 11).
Dow Process:
Degreasexe2x86x92Cathodic Cleaningxe2x86x92Acid Picklexe2x86x92Acid Activationxe2x86x92Zincatexe2x86x92Cu Plate.
Norsk-Hydro Process:
Degreasexe2x86x92Acid Picklexe2x86x92Alkaline Treatmentxe2x86x92Zincatexe2x86x92Cu Plate
WCMProcess:
Degreasexe2x86x92Acid Picklexe2x86x92Fluoride Activationxe2x86x92Zincatexe2x86x92Cu Plate
The Dow process was the first to be developed but has been shown to give uneven zinc distributions as well as poor adhesion in many cases. A modified version of the Dow process (13) introduces an alkaline activation following the acid activation step. This results in good adhesion of Nixe2x80x94Au films on AZ31 and AZ91 alloys. The authors shortened the pretreatment time, which is important in a manufacturing setting. The Norsk-Hydro process has been shown to improve the quality of the zinc coating on AZ61 alloy in terms of adhesion, corrosion resistance and decorative appearance. Deposits of Cuxe2x80x94Nixe2x80x94Cr, on samples pretreated with this process, have been shown to exceed the standards for outdoor use (see references 14a and 14b). Dennis et al. (11, 15) show that samples treated with both the Dow and Norsk-Hydro processes give porous zinc coatings and perform poorly in thermal cycling tests. It was found that the WCM process resulted in the most uniform zinc film and was the most successful in terms of adhesion, corrosion and decorative appearance. However, preferential dissolution of magnesium rich areas on the alloys occurred with all 3 processes, which could limit the effectiveness of any of these pretreatment methods.
A similar process has been used as an undercoating for samples to be plated with a series of metals by electroless and electroplating techniques (16). A slight variation of the pretreatment uses a copper cyanide plating bath that contains a soluble silicate (17). Zinc immersion prior to tin plating of magnesium has also been explored (18). A magnesium alloy is treated with a conventional zinc immersion pretreatment and then zinc plated in an aqueous zinc pyrophosphate bath. Tin is subsequently plated to improve the tribological properties of the plated alloy.
As stated above, the disadvantages of the direct electroless nickel plating methodology include the short lifetime of the plating baths, the toxic chemicals used in the pretreatment process, and the narrow operating window required to achieve optimum coatings.
The zinc immersion process has the disadvantages of poor uniformity of the zinc undercoating, and the need for extremely toxic chemicals in the copper cyanide strike prior to any further plating.
A two-step coating process (19) was reported to be applicable for the coating of magnesium and its alloys: the first step is an immersion coating process and the second step is an electroless deposit as the topcoat. However, the claimed immersion process can only produce a semi-continuous coating, which is not preferable as a coating. The non-continuous nature of the coating will, in fact, accelerate the corrosion of magnesium rather than protecting it in the event of a topcoat failure. The process described in reference 19 was only exemplified with aluminium rather than on magnesium alloys.
A need exists for a process capable of providing a uniform coating on magnesium or magnesium alloy materials having complex geometric shapes. Further, a need exists for such a process that minimizes the use of toxic chemicals and is not line-of-sight dependent.
It is an object of the present invention to provide a process for chemically coating magnesium and its alloys, which process obviates or mitigates at least one disadvantage of previous coating processes.
In a first aspect, the present invention provides a process for coating an object formed of magnesium or a magnesium alloy comprising the steps of: immersion coating the object in a sonicated bath to form an undercoat, and subsequently topcoating the object to form a topcoat, wherein the undercoat is more noble than or equally noble as the topcoat. The topcoating step may comprise electroless deposition, or any other known coating process. This aspect of the invention is particularly advantageous if there is potential for topcoat failure. If damage is done to the topcoat, exposure of the less reactive undercoat would not result in corrosion or reactivity of the undercoat.
In a further aspect, the present invention comprises a process for coating an object formed of magnesium or a magnesium alloy comprising the steps of: immersion coating the object in a sonicated bath to form an undercoat, and topcoating the object. This aspect of the invention does not necessarily require a particular nobility gradient between the topcoat and the undercoat, provided that topcoat failure is unlikely to occur. For instance, where there is little likelihood of damage to the topcoat, the topcoat could be more noble than the undercoat.
The invention encompasses embodiments wherein the topcoating step is conducted using a process selected from the group consisting of electroless deposition, electroplating, brush plating, powder coating and a combination thereof.
According to embodiments of the invention, the object to be coated may be formed of a magnesium-based alloy, such as but not limited to the group consisting of AZ91, AM60, AZ31, WE54, ZE63, ZK21, and ZM21.
Further, the topcoat may comprise a metal selected from the group consisting of Ni, Ti, Mn, Al, Fe, Co, Zr, Mo, Nb and W. The topcoat may be a metal alloy or a metal composite.
Advantageously, the inventive process for coating magnesium and its alloys is not line-of-sight dependent and is therefore capable of providing uniform coatings on the entire surface of shaped objects having a complex geometry including sharp corners, edges and deep pockets.